Advanced structural composites are high modulus, high strength materials useful in many applications requiring high strength to weight ratios, e.g., applications in the automotive, sporting goods, and aerospace industries. Such composites typically comprise reinforcing fibers (e.g., carbon or glass) embedded in a cured resin matrix.
A number of the deficiencies of advanced composites result from limitations of the matrix resins used in the fabrication of the composites. Resin-dependent properties include composite compression strength and shear modulus (which are dependent on the resin modulus) and impact strength (which is dependent on the resin fracture toughness). Various methods of improving these resin-dependent composite properties have been attempted. For example, elastomeric fillers (such as carboxyl-, amino-, or sulfhydryl-terminated polyacrylonitrile-butadiene elastomers) have been incorporated, thermoplastics (such as polyether imides or polysulfones) have been incorporated, and the crosslink density of the matrix resin has been decreased by using monomers of higher molecular weight or lower functionality. Such methods have indeed been effective at increasing resin fracture toughness and composite impact strength. But, unfortunately, the methods have also produced a decrease in the resin modulus and, accordingly, a decrease in the compression strength and shear modulus of composites made from the resins. (And the methods have tended to degrade the high temperature properties of the composites, as well.) Thus, composites prepared by these methods have had to be thicker and therefore heavier in order to exhibit the compressive and shear properties needed for various applications.
Other methods have focused on increasing the modulus of matrix resins as a means of increasing composite compressive and shear properties. For example, "fortifiers" or antiplasticizers have been utilized. Such materials do increase the modulus of cured epoxy networks but also significantly reduce glass transition temperature and increase moisture absorption. Thus, the materials are unsatisfactory for use in high performance composite matrix resins.
Conventional fillers (fillers having a particle size greater than one micron) can also be used to increase the modulus of cured thermosetting resin networks, but such fillers are unsuitable for use in the fabrication of advanced composites for the following reasons. During the curing of a fiber-containing composite composition, resin flow sufficient to rid the composition of trapped air (and thereby enable the production of a composite which is free of voids) is required. As the resin flows, finer denier fibers can act as filter media and separate the conventional filler particles from the resin, resulting in a heterogeneous distribution of filler and cured resin which is unacceptable. Conventional fillers also frequently scratch the surface of the fibers, thereby reducing fiber strength. This can severely reduce the strength of the resulting composite.
Amorphous silica microfibers or whiskers have also been added to thermosetting matrix resins to improve the impact resistance and modulus of composites derived therefrom. However, the high aspect ratio of such microfibers can result in an unacceptable increase in resin viscosity, making processing difficult and also limiting the amount of microfiber that can be added to the matrix resin.
Accordingly, there is a need in the art for methods of producing matrix resin systems that are high in both fracture toughness and modulus, and which therefore provide composites exhibiting high toughness as well as high compressive and shear properties. Such methods should also maintain the low viscosity and easy processability of conventional resin systems.